create a gwaa.data-class object from the data frames in a Mega2 environment. This
function is a front end that eventually calls a C++ Rcpp function that reads
the genotype data in Mega2 compressed format and converts it to the GenABEL
compressed format. The results of Mega2ENVGenABEL are/should be the same
as Mega2GenABEL, but the calculation is much faster, typically a factor of 10 to 20.